The invention relates to a viewfinder system for photographic cameras, and more particularly to a focusing screen forming part of the viewfinder system. The present invention is useful particularly but not exclusively in the viewfinder system of a mirror reflex camera.
In such a camera, a focusing or viewfinder screen is arranged in a plane which is conjugate to the image plane, allowing the image formed on the focusing screen to be focused exactly and without any interference. A number of widely varying focusing screens for this purpose are known in the art, such as clear glass screens, ground glass screens, micro-screens, and screens with split field centers. Various types of screens are used for various applications. For example, one type of screen may be preferred for microscope photography, another type for astro photographs, perhaps another type for macroshots, another for photographs in poor light conditions, or photographs in conjuction with long focus lenses. To insure the versatility of a camera, the focusing screen of the viewfinder system is frequently designed to be interchangeable, so that it may be removed from the camera and replaced by another screen of different character, when a different type of photography is to be performed.
The making of the camera in a way to permit easy interchangeability of the focusing screen entails not only constructional problems in many cases, but also an increased cost. Hence exchangeable focusing screens are usually found only in cameras of relatively high price. In addition to this, the exchanging of screens is usually a fairly complicated and time-consuming operation, which entails the additional disadvantage of having to store and carry the desired interchangeable screens.
An important object of the present invention, therefore, is to design a viewfinder system, including the focusing screen, in such a way that it may be used in a plurality of different modes without changing the screen, thus enabling the photographer to have the advantages of two or more different types or kinds of screen for selective use, without actually changing the screen in the camera. The photographer may thus adapt his focusing screen, within certain limits, to the respective photographic conditions and purposes of the moment, with no need to exchange the focusing screen.